Sudden cardiac death occurs in as many as 400,000 people every year in the U.S. and is primarily due to the sequela of ventricular tachycardia followed by ventricular fibrillation or primary ventricular fibrillation. These life threatening ventricular tachyarrhythmias have been treated with anti-arrhythmic drugs and/or implantable defibrillators with variable success. It is our primary hypothesis that the key to an effective interventional strategy for reducing sudden cardiac death directly depends on an accurate noninvasive approach to risk stratification. This diagnostic step will largely determine the clinical efficacy of the available therapies and optimize resource allocation in a cost conscious medical delivery environment. There are several ECG based parameters which identify high risk patients with varying degrees of success. The measurement of cardiac late potentials, heart rate variability, T wave alternans, QT dispersion, ventricular ectopy, and ST segment shifts are examples of these noninvasive tests. Each of these measurements is aimed at quantifying some aspect of the arrhythmia mechanism. The goal of this grant is to develop new analytic methods for quantifying several noninvasive parameters in the context of arrhythmia mechanisms with the outcome being a highly accurate approach to risk stratification. A noninvasive method for identifying the mechanism of specific spontaneous ventricular arrhythmias would directly affect clinical management decisions resulting in the improved medical care and the optimal use of resources. We plan to develop a unified approach for long term ECG recording and analysis using a single recording technology which would provide the basis of all the measurements. This will consolidate a number of tests with a simplified and improved device providing an economical approach to risk stratification. The specific aims of this grant are: 1) Determine the parameters of conventional and intra-QRS cardiac late potentials which are correlated with spontaneous ventricular arrhythmias. 2) Develop new measures of the repolarization phase (QT dispersion, T wave alternans, and the T/U wave region) of the ECG in the context of spontaneous ventricular arrhythmias. 3) Measure the sub-clinical changes (<0.1 mVolt) in the ST segment and correlate this with the occurrence of spontaneous ventricular arrhythmias. 4) Correlate the new ECG indices, retrospectively, in two patient groups: a) those with complex ventricular arrhythmias and/or an existing ICD and b) those with a new or recent myocardial infarction.